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Tactical Rifle: Ruger SR-556

The last time I wrote about the AR-15 rifle platform my biggest design complaint was the Direct Gas Impingement (DGI) action. This has been a feature since the very first generation of the weapon adopted by the military over 45 years ago. DGI contributes to the platform’s simplicity and lightness, but also has some undesirable consequences: Every time the gun fires it blasts hot propellant gasses back into the bolt carrier, baking a layer of carbon onto everything in the receiver. Decades of military service have proven that well-made AR-15’s can withstand this abuse and continue to function reliably with just basic cleaning and lubrication. But for obsessive owners who like to keep their guns in more immaculate condition this dirty design can be quite irritating.

When I bought my first AR-15 following the 2004 expiration of the federal “Assault Weapons” Ban I openly wondered why no manufacturer had produced a piston-driven variant of the gun, especially given the huge market for America’s favorite tactical rifle. Since then I tracked the gradual emergence of AR-15 piston actions, first as retrofit kits, then as new uppers and complete guns. There was a great deal of concern about the long-term reliability of converted actions, mostly because the AR-15 bolt carrier wasn’t designed to be hammered with a piston, and its geometry seemed prone to an adverse effect called “bolt carrier tilt.” Some manufacturers, most notably LWRC, built a new action from scratch to mitigate these concerns. However LWRC’s piston uppers were always very hard to acquire, and even today they command huge premiums.

In mid 2009 Ruger entered the AR-15 market for the first time, and they did so with a proprietary piston action. The SR-556 now represents the best value in piston-driven AR-15s. I sold my old DGI AR-15 to somebody less fastidious than me and last month managed to pick up a new SR-556 for just under $1500.

The SR-556 comes standard with a number of desirable upgrades over basic AR-15s. Among them:

Full-length quad-rail handguards, so you can easily add as many accessories as you can carry.

Top-quality folding iron sights (by Troy Industries).

A comfortable pistol grip (by Hogue).

Chrome-plated bolt carrier assembly, so what little dirt does get into the action can easily be wiped off.

The piston system was executed superbly. Only the three pieces in the front two inches need to be removed for cleaning. The piston regulator has four positions. In the off position the action doesn’t cycle, essentially giving you a bolt-action gun. The first position (smallest gas opening) is suited to use with silencers, which sustain more backpressure on the action. The second position cycles the action reliably under normal conditions, and the largest opening can be used to compensate for weak ammunition, a very dirty gun, or other harsh conditions.

Shooting the gun for the first time was a pleasure. There were absolutely no malfuctions. Even after sustained firing the bolt was cool to the touch (though the barrel and gas vent area still got quite hot!). Long-term reliability on a new design may not be certain, but the fact that Ruger is comfortable enough to roll these out in quantity is reassuring. This should certainly outlast piston systems that use DGI bolt carriers with their separate gas key: The SR-556 carrier is machined from a single piece of steel with just a little hump over the main tube to take the impact of the piston.

Out of the box this is a very good gun, but it does have a few shortcomings:

Like all piston guns this is both front-heavy and heavier (just under 8 pounds) than comparable DGI AR-15s (typically closer to 7 pounds). Of course even professional users of the DGI M4s have found it beneficial to put a vertical grip on the front of their guns. I added a GripPod ($80 from Botach) and found this mitigates the imbalance nicely, in addition to deploying a handy bipod when desired.

The flash suppressor has uniform slots around its entire diameter. When shooting prone or on a table this causes some of the muzzle blast to kick up dust. Although closed-bottom flash suppressors require indexing to install, this should be a trivial factor to accommodate on a production line. So shame on Ruger for not equipping this with a closed-bottom flash suppressor!

The telescoping buttstock is a standard M4 style, which leaves a lot of play between the stock and the buffer tube. They should have just equipped it from the start with a Magpul CTR (shown in the photo above), for which I had to pay another $80. (To fit this gun be sure to get the mil-spec, not commercial size CTR!)

First, he notes that the advantage in terms of cleanliness is diminished when shooting with a suppressor:

I only run the guns with suppressors for testing when I did my comparison, and with suppressors, direct-impingement and piston-operated were both very dirty, ’cause the blowback comes to the chamber, not the gas tube. … On a piston gun or gas-impingement, the case is being extracted while the suppressor is still under pressure. Now you have all the pressure in that suppressor exiting both out the front and the back.

Also, we should mention the poor choice of platform for the piston conversion on a round receiver bore as found on the M16/M4 system. All other piston type systems out there utilize a railed receiverdesign, like the M14, AK-47, M249, FAL and so on. The round receiver bore design used on the M4 is only acceptable for the standard op system. The carrier and bolt expand on axis with the bore under the normal gas impingement cycle, but on a pistongun , you run into off center impulse issues with carrier tilt and incorrectly designed carrier contact points. Some designs attempt to address the carrier tilt problem with over sized carrier tails and rollers. I do not believe the receiver extension should be used in this manor. I know many people are very happy with their piston weapons. This is not meant as a knock on the piston conversion systems out there, but as a philosophical dialogue focused the new physiological relationships applied to the M16/M4 platform through the introduction of an operating system which has traditionally been applied to receivers with rails for the bolt and/or carrier. I would rather see an entirely new weapon system designed for the piston from the ground up.